The present invention relates to an improved boost converter, particularly suited for directly driving a capacitive load at a relatively high voltage. The circuit is particularly suited for integration, being capable of delivering a voltage that is substantially twice the intrinsic breakdown voltage of the integrated structures that would compose the circuit.
The necessity of driving or powering components such as electroluminescent diode displays (EL LAMPS) and more in general low power actuators or indicators functioning at relatively high voltage, for example at voltages comprised between 60 V and 120 V, is often encountered in electronic apparatuses.
Reduction of energy consumption is an attendant requisite that becomes of fundamental importance in battery operated portable apparatuses, for example electronic watches, pocket calculators, computers, etc.
The use of voltage multipliers for generating high voltages, starting from particularly low supply voltages, for example 1.2 V battery voltage, is practically impossible because of a progressive increase of losses with the number of stages. In these cases, DC--DC converter circuits commonly referred to as boost converters, are employed.
A boost converter is functionally constituted by an energy storing inductor, connected to the supply and momentarily driven toward ground by a switch, during a charging phase. The energy stored in the inductor is then discharged during a successive discharge phase through a diode to an output capacitor, across the terminals of which an incrementally increased voltage may be developed in function of the number of cycles of discharge of the reactive energy stored in the inductor. Normally a boost converter is completed by a control circuit that regulates the voltage developed across the charge storing capacitor by appropriately driving the switch. The charge capacitor or capacitors provide a high voltage supply to user circuits or to components that normally require a high voltage.
In case of components that may be driven at a high voltage and which may be electrically assumed to be equivalent to a capacitance having a more less high loss-factor, as for example electroluminescent diodes, it is a common practice to directly drive the load element with the converter circuit, by inverting, by the use of pairs of switches, driven in phase opposition by complementary driving signals at a relatively low frequency, the connections of the two terminals of the load (that may be considered as a capacitor) to the cathode of the discharging diode of the inductor and ground.
As an example, such a direct driving circuit for an electroluminescent diode is disclosed in U.S. Pat. No. 4,527,096. By fixing the number of switching pulses that are fed to the control terminal of the switch that is used to charge the inductor, during a phase of configuration of the connections of the electroluminescent diode, it is possible to predefine the maximum peak-to-peak voltage that is applied to the capacitive load at the end of each configuration phase. In practice, a succession of stepped ramps of voltage of alternating sign are applied to the load. (The load device would preferably include an internal switching or diode network, in order to make use of both the high voltage and low voltage peaks provided.)
The converter is capable of providing a high voltage boost ratio with small energy losses and therefore with a high deficiency, an aspect of fundamental importance in battery powered applications.
Such a DC--DC boost converter circuit may be realized by employing discrete components andr employing electromechanical relays for implementing the switches necessary for cyclically inverting the load connections. Alternatively, the converter circuit may be realized in the form of an integrated circuit, by employing electronic switches in the form of bipolar or field effect transistors with obvious advantages in miniaturization capabilities.
This possibility is disclosed in U.S. Pat. No. 4,527,096, wherein it is remarked the need of implementing the integrated circuit with a fabrication process for high voltage devices, in view of the fact that the relative integrated structures must be capable of withstanding a reverse voltage equal to the maximum-to-peak voltage that is generated by the boost converter across the capacitive load. In other words, the integrated circuit must have breakdown characteristics that are sufficiently higher than the value of the maximum peak-to-peak voltage produced by the circuit.
It is known that the architecture of integrated circuits may be adapted for achieving particularly high breakdown voltages as imposed by design needs, and which in the specific case of a boost converter circuit that can be used for directly driving a capacitive load, as the one represented by an electroluminescent diode, may reach exceptionally high values. This requirement, though not precluding the possibility of realizing the boost converter in the form of a monolithically integrated circuit, imposes the realization of sufficiently sturdy integrated structures for withstanding a reverse voltage that may reach up to about 120 V. This imposes adequate sizes, thicknesses, depth of junctions and/or the implementation of particular structures and arrangements suitable to increase the breakdown voltages. In general these requisites determine limits to the miniaturization of the integrated circuit itself, which normally contains the whole functional circuit of the apparatus, beside the boost converter for driving an external load.
Therefore there exists a need or utility for a boost converter, suitable for directly driving at a relatively high voltage a capacitive load, having a simplified circuit and which at the same time is readily integratable and, in such a monolithically integrated form, also permits a marked miniaturization of the whole integrated circuit.
The boost converter circuit for driving a capacitive load object of the present invention employs four switches for delivering across the terminals of a capacitive load, an alternating sequence of positive and negative voltage ramps, at a first control frequency. Each voltage ramp is produced through a certain number of charge and discharge of an inductor on the capacitive load, at a second switching frequency.
A control circuit generates the control signals at said first control frequency and at said second switching frequency. The first control frequency may be in the order of 20-100 hertz (Hz), while the second switching frequency may be in the order of kilohertz (kHz).
In practice, each terminal of the inductor is connected to a charging switch and to a discharge path that comprises a switch and a diode in electrical series with each other, connected between the terminal of the inductor and the output node. The two discharge paths are alternately enabled by a pair of complementary signals at said first frequency, which close one or the other of the two switches that enable the respective discharge path toward the output node to which an external capacitive load is connected.
Coherently, a charging switch of the inductor is driven to switch at a relatively high frequency (said second switching frequency) while another charging switch remains open during the entire semiperiod. During the following semiperiod, the circuit configuration is reversed, so as to produce a ramp of opposite polarity to the preceding ramp.
The voltage ramps that are produced have a high frequency ripple, corresponding to the switching frequency of the respective charging switch of the inductor.
It should also be noted that the disclosed circuit configuration is novel aside from the timing considerations discussed (i.e. the operation of switches A and B at a much higher peak frequency than the frequency of operation of switches C and D).